The disclosure relates generally to methods for protection of ceramic coatings and maintenance and inspection of such coatings.
Ceramic coatings such as thermal barrier coatings (TBCs) are typically used in components that operate at or are exposed to high temperatures. Aviation turbines and land-based turbines, for example, may include one or more components protected by ceramic coatings, including thermal barrier coatings. Under normal conditions of operation, coated components may be susceptible to various types of damage, including erosion, oxidation, and attack from environmental contaminants Current trends in the industry toward higher operating temperatures to improve efficiency are pushing component materials to extend their capability to operate in ever more aggressive environments.
A conventional coating system applied in turbine applications typically includes a ceramic coating (e.g., a thermal barrier coating (TBC)) disposed on a nickel-based superalloy substrate component (often with internal cooling passages), often including a metallic bond coating of either platinum-nickel-aluminide or MCrAlY (where M includes Ni, Co, Fe, or mixed combination) interposed between the substrate and the ceramic coating. Zirconia stabilized with yttria, known in the art as yttria-stabilized zirconia, or YSZ, is the most commonly used material for the ceramic coating. There is interest in developing new thermal barrier coating systems through microstructural and/or compositional changes of the YSZ to improve the life of turbine components.
For turbine components, environmental contaminant compositions of particular concern are those containing oxides of calcium, magnesium, aluminum, silicon, and mixtures thereof. Dirt, ash, and dust ingested by gas turbine engines, for instance, are often made up of such compounds. These oxides often combine to form contaminant compositions comprising mixed calcium-magnesium-aluminum-silicon-oxide systems (Ca—Mg—Al—Si—O), hereafter referred to as “CMAS.” At turbine operating temperatures, which are high temperatures, these environmental contaminants can adhere to the hot thermal barrier coating surface, and thus cause damage to the thermal barrier coating. For example, CMAS can form compositions that are liquid or molten at the operating temperatures of the turbines. The molten CMAS composition can dissolve the thermal barrier coating, or can fill its porous structure by infiltrating the voids, cracks, channels, columns, pores, or cavities in the coating. Upon cooling, the infiltrated CMAS composition solidifies and reduces the coating strain tolerance, thus initiating and propagating cracks that may cause delamination and spalling of the coating material. This may further result in partial or complete loss of the thermal protection provided to the underlying metal substrate of the part or component. Further, spallation of the thermal barrier coating may create hot spots in the metal substrate leading to premature component failure. Premature component failure can lead to unscheduled maintenance as well as parts replacement resulting in reduced performance, and increased operating and servicing costs.
There is a need in the field for methods and materials that prevent and/or reduce damage to thermal barrier coatings and that allow for easy maintenance of thermal barrier coatings.